Serna was with the U-20s at the recently completed Toulon Tournament in France, where he appeared in three of the U-20's four matches including a start in the 2-1 loss to Colombia. And while he's on his way back to Colorado, he is one of 14 players that can still be called in for the World Cup in the event something happens with one of the rostered players.

In mid-May, the U.S. submitted a 35-player provisional roster for the World Cup, from which Ramos could select the final squad. The U.S. can make roster changes up to 24 hours before the start of the tournament.

“It’s very important for all of them to stay fit and to be prepared to get a call,” Ramos said. “We’ve already spoken with some players who are not going with us to Portugal (the team’s training base for the next week) who are not on the 21 and we’ve made them aware of the fact that they might get a last-minute call.”